1. Field of the Invention
The present invention relates to a cellular mobile radio communication system of a type wherein each of radio zones is covered by corresponding radio base stations, and particularly to a mobile radio communication system having radio zones in the form of sector configurations, wherein each radio zone has a plurality of antennas whose horizontal-plane half-power beam widths are different and connected directly to the radio base stations so that the radio zones are divided into sector cells corresponding to directivities of the antennas.
2. Description of the Related Arts
A mobile radio communication system such as a cellular mobile telephone system uses the same radio channel (hereinafter abbreviated as "channel") in each radio zone as far as it does not cause interference. In this type of mobile radio communication system, omni and sector zone configurations are known as the configurations of the radio zones. In the omni zone configuration, an antenna having a single horizontal plane non-directional antenna in the horizontal plane, i.e., an omnidirectional antenna is provided for a radio base station. The omnidirectional antenna covers a single radio zone in which the radio base station corresponding to the omnidirectional antenna is provided at the center. On the other hand, in the sector zone configuration, one radio base station is provided with a plurality of sector antennas each having an individual horizontal-plane radiation pattern, respectively. Further, each sector antenna covers a sectional region (hereinafter abbreviated as "sector cell") corresponding to the directivities of the sector antennas. In the sector zone configuration, since interference in the same channel is scarce, owing to the limited direction effects of the sector antennas, therefore, the distance limitation in using the same channel is shorter as compared with the omni zone configuration and the efficiency of utilization of a frequency or a channel is high. The aforementioned two configurations of radio zones have been described in detail in the article ("Radio Zone Configuring Method suitable for Car Telephone" by Yoshikawa, Nomura, Watanabe and Nagatsu, Research Practical Report, Vol. 23, No. 8, 1974). In the mobile radio communication system, a plurality of fixed or mobile radio terminals are included in the plurality of radio zones. These radio terminals communicate with other radio terminals or terminals in a public communication network through radio channels between radio base stations which belong to the radio zones in which the radio terminals exist and the radio terminals.
As typical sector configurations, a three-sector configuration comprised of three sector cells per radio base station and a six-sector configuration comprised of six sector cells per radio base station are well known However, a sector zone configuration is also considered wherein directional antennas in which each half-power beam width angle (angular width formed between two directions in which a directional gain or directivity falls 3dB in both directions from the central direction having maximum radiation intensity) has a small overlap in the three-sector configuration. Since it is easy to increase the directional gain or directivity of the directional antennas whose half-power beam width angles are small as compared with a non-directional antenna, such a sector configuration is used to cover a local service area distant from the radio base station without providing new radio base stations. A reduction in interference among upward lines (lines which radio terminals use for transmitting to the radio base station whereas the radio base station uses them for receiving) and a reduction in interference among downward lines (lines which the radio base station uses for transmitting whereas the radio terminals use them for receiving) can be achieved by using the directional antenna whose half-power beam width angles is small. Further, the same frequency can be simultaneously and repeatedly used among the nearer cells by using the smaller half power beam width angle. Accordingly, the configuration of the sector cells narrow in width overlapping the normal sector configuration such as the three-sector configuration or the like is employed in particular to furnish communications services to a concentrated area of traffic requiring assignment of a number of channels.
In a zone configuration in which a plurality of cells are overlapped using a plurality of antennas so as to cover the same area as in the case where the configuration of the sector cells narrow in width to overlap the normal sector configuration as described above, a plurality of antennas usable for communicating with a radio terminal are provided. It is thus necessary to select an antenna to be used for communication at this time.
When a plurality of sector antennas which belong to a plurality of radio base stations and the same radio base station that are usable for communicating with a radio terminal in the conventional mobile radio communication system, a received level (corresponding to the level of a desired radio wave) of a signal sent from the radio terminal is measured through each respective sector antenna. Thereafter, a sector antenna providing the maximum receiving levels is selected to perform communication between the radio terminal and its corresponding radio base station.
This method has, however, a problem in that an effect for improving the efficiency of utilization of a frequency owing to a reduction in interference by virtue of the directivity of an antenna cannot be sufficiently obtained. The reason for this is as follows. Namely, when the level of the desired radio wave received by the sector antenna whose half-power beam width of directivity is large is slightly larger than that of the desired radio wave received by the sector antenna whose half-power beam width angle is small, the conventional method of selecting the antenna capable of providing the maximum level of desired radio wave selects the sector antenna whose half-power beam width angle is large in spite of the fact that the selected sector antenna whose half-power beam width is small permits an improvement in the effect of reducing interference by the antenna directivity.
Therefore, a method of preferentially selecting a sector antenna whose half-power beam width angle is small at all times has been considered to cope with such a case.
In order to control or cut back power consumption of the radio terminal and improve the efficiency of utilization of the frequency by virtue of the reduction in interference with the upward line, a method of controlling power to be transmitted from the radio terminal so that the received level of the desired radio wave at the corresponding radio base station is kept substantially constant, has been adopted in some mobile radio communication systems. Thus, such a system enables a reduction in the transmitting power of a radio terminal so long as a sector antenna capable of securing a desired radio wave having a large level is selected upon selecting a sector antenna used for communication. However, the method of preferentially selecting always the sector antenna whose half-power beam width is small causes problems that since the sector antenna whose half-power beam width is small is selected so long as the received level of the desired radio wave is larger than the limit of allowable value for connection between the radio terminal and its corresponding radio base station even when the level of the desired radio wave received by the sector antenna whose half-power beam width angle is small is less than that of the desired radio wave received by the sector antenna whose half-power beam width is large, the radio terminal is required to increase the transmitting power, whereupon an effect for cutting back the power consumption of the radio terminal controlling the power for transmission and improving the efficiency of utilization of the frequency owing to the reduction in interference among upward lines cannot be sufficiently obtained.